This invention relates to nuclear reactors and, more particularly, to fuel arrangements in a reactor core. A major objective of the present invention is to provide for more thorough fuel burnups to enhance fuel utilization and minimize active waste products.
Fission reactors rely on fissioning of fissile atoms such as uranium isotopes (U233, U235) and plutonium isotopes (Pu239, Pu241). Upon absorption of a neutron, a fissile atom can disintegrate, yielding atoms of lower atomic weight and high kinetic energy along with several high-energy neutrons. The kinetic energy of the fission products is quickly dissipated as heat, which is the primary energy product of nuclear reactors. Some of the neutrons released during disintegration can be absorbed by other fissile atoms, causing a chain reaction of disintegration and heat generation. The fissile atoms in nuclear reactors are arranged so that the chain reaction can be self-sustaining.
To facilitate handling, fissile fuel is typically maintained in modular units. These units can be assemblies of vertically extending fuel rods. Each rod has a cladding which encloses a stack of fissile fuel pellets. Generally, each rod includes a space or "plenum" for accumulating gaseous byproducts of fission reactions which might otherwise unacceptably pressurize the rod and lead to its rupture. The assemblies are arranged in a two-dimensional array in the reactor. Neutron-absorbing control rods are inserted between or within fuel bundles to control the reactivity of the core. The reactivity of the core can be adjusted by incremental insertions and withdrawals of the control rods.
Both economic and safety considerations favor improved fuel utilization, which can mean less frequent refuelings and less exposure to radiation from a reactor interior. In addition, improved fuel utilization generally implies more complete fuel "burnups".
Variations in neutron flux density, which occur along the length of a bundle, make it difficult to achieve complete burnups. For example, fuel near the top or bottom of a fuel bundle is subjected to less neutron flux than is fuel located midway up a fuel bundle. These axial variations are not effectively addressed by radial redistribution of fuel elements.
In addition to the variations in neutron flux density, variations in spectral distribution affect burnup. Initially, neutrons released during fissioning move too quickly and have too high an energy to readily induce the further fissioning required to sustain a chain reaction. These high energy neutrons are known as "fast" neutrons. Slower neutrons, referred to as "thermal neutrons", most readily induce fission.
Dual-phase reactors store heat generated by the core primarily in the form a phase conversion of a heat transfer medium from a liquid phase to a vapor phase. The vapor phase can used to physically transfer stored heat to a turbine and generator, which are driven to produce electricity. Condensate from the turbine can be returned to the reactor, merging with recirculating liquid for further heat transfer and cooling. The primary example of a dual-phase reactor is a boiling-water reactor (BWR). Dual-phase reactors are contrasted with single-phase reactors, which store energy primarily in the form of elevated temperatures of a liquid heat-transfer medium, such as liquid metal. The following discussion relating to BWRs is readily generalizable to other forms of dual-phase reactors.
In BWRs, thermal neutrons are formerly fast neutrons that have been slowed primarily through collisions with hydrogen atoms in the water used as the heat transfer medium. Between the energy levels of thermal and fast neutrons are "epi-thermal" neutrons. Epithermal neutrons exceed the desired energy for inducing fission but promote resonance absorption by many actinide series isotopes, converting some "fertile" isotopes to "fissile" (fissionable) isotopes. For example, epithermal neutrons are effective at converting fertile U238 to fissile Pu239. Within a core, the percentages of thermal, epithermal, and fast neutrons vary over the axial extent of the core.
Axial variations in neutron spectra are caused in part by variations in the density or void fraction of the water flowing up the core. In a boiling-water reactor (BWR), water entering the bottom of a core is essentially completely in the liquid phase. Water flowing up through the core boils so most of the volume of water exiting the top of the core is in the vapor phase, i.e., steam. Steam is less effective than liquid water as a neutron moderator due to the lower density of the vapor phase. Therefore, from the point of view of neutron moderation, core volumes occupied by steam are considered "voids"; the amount of steam at any spatial region in the core can be characterized by a "void fraction". Within a fuel bundle, the void fraction can vary from about zero at the base to about 0.7 near the top.
Continuing the example for the BWR, near the bottom of a fuel bundle, neutron generation and density are relatively low, but the percentage of thermal neutrons is high because of the moderation provided by the low void fraction water at that level. Higher up, neutron density reaches its maximum, while void fraction continues to climb. Thus, the density of thermal neutrons peaks somewhere near the lower-middle level of the bundle. Above this level, neutron density remains roughly stable while the percentages of epithermal and fast neutrons increase. Near the top of the bundle, neutron density decreases across the spectrum since there are no neutrons being generated just above the top of the bundle.
The inhomogeneities induced by this spectral distribution can cause a variety of related problems. Focusing on the upper-middle section, problems of inadequate burnup and increased production of high-level waste are of concern. Since the upper-middle section has a relatively low percentage of thermal neutrons, a higher concentration of fissile fuel is sometimes used to support a chain reaction. If the fuel bundle has a uniform fuel distribution, this section could fall below criticality (the level required to sustain a chain reaction) before the other bundle sections. The fuel bundle would have to be replaced long before the fissile fuel in all sections of the bundle were depleted, wasting fuel. While it is possible to disassemble a spent fuel bundle to recover unspent fuel, this is much more expensive and complicated than using fuel while it is in the bundle.
The problem with waste disposal is further aggravated at this upper-middle section since the relatively high level of epithermal neutrons results in increased production of actinide-series elements such as neptunium, plutonium, americium, and curium, which end up as relatively long half life, high-level waste. For example, about 1% of the U238 in a fuel bundle is converted to plutonium, mainly Pu239, along with Pu240, Pu241 and Pu242. Pu239 and Pu240 have long halflives so that considerable expense is incurred if these isotopes remain unburned and long term protection and storage are required.
One method of dealing with axial spectral variations is using a control rod. For the BWR, control rods typically extend into the core from below and contain neutron-absorbing material which robs the adjacent fuel of thermal neutrons which would otherwise be available for fissioning. Thus, control rods can be used to modify the distribution of thermal neutrons over axial position to achieve more complete burnups. However, control rods provide only a gross level of control over spectral density.
More precise compensation for spectral variations can be implemented using enrichment variation and burnable poisons. Enrichment variation using, for example, U235 enriched uranium, can be used near the top of a fuel bundle to partially compensate for a localized lack of thermal neutrons. Similarly, burnable poisons such as gadolinium oxide (Gd.sub.2 O.sub.3), can balance the exposure of bundle sections receiving a high thermal neutron flux. Over time, the burnable poisons are converted to isotopes which are not poisons so that more thermal neutrons become available for fissioning as the amount of fissile material decreases. In this way, fissioning can remain more constant over time in a section of the fuel bundle. By varying the amount of enrichment and burnable poisons by axial position along a bundle, longer and more complete burnups can be achieved. In addition, the enrichment and poison profiles can be varied by radial position to compensate for radial variations in thermal neutron density.
Fuel management using control rods, selective enrichment and burnable poisons can be used to control the void fraction within the core as a function of time. Controlling the void fraction over time, in turn, results in control over the neutron flux profile. Thus, early in the life cycle of a fuel element, a large void fraction can be implemented, resulting in high heat generation at the bottom of a fuel bundle, while conversion of fertile fuel is facilitated over most of the fuel bundle length. Over time, the void fraction can be reduced, so that the portion of the fuel bundle subjected to thermal neutrons is increased, promoting further burnup at successively higher levels within the bundle that have been enriched by the earlier conversion.
Imposing a progressively diminishing void fraction permits more complete burnups. However, it requires non-uniform power distributions. Non-uniform power distributions require lower reactor outputs, since peak temperatures must remain within limits to prevent excessive outgassing of fission products within fuel rods. Furthermore, non-uniform power distributions can induce additional thermal stresses along fuel rods; these stresses must be managed by limiting reactor output power. Moreover, void fractions can be controllably varied by adjusting pumping rates and thus coolant flow. The ability to control void fractions by increasing coolant flow is much more limited in some types of reactors. For example, natural-circulation boiling-water reactors rely on convection rather than pumps to promote circulation.
Taken together, the use of control rods, control of void fraction over time, radial positional exchange of fuel assemblies, selective enrichment and distribution of burnable poisons still leave problems with axial variations in burn rates and neutron spectra. Furthermore, none of these employed methods effectively addresses the problem of the high level of fissile material produced and left in the upper-middle sections of the bundle due to the high level of epithermal neutrons and the low level of thermal neutrons. What is needed is a system that deals more effectively with axial spectral variations in neutron flux so that higher fuel burnups are provided and so that high-level waste is minimized.